Selective catalytic reduction catalyst system

ABSTRACT

Described are SCR catalyst systems comprising a first SCR catalyst composition and a second SCR catalyst composition arranged in the system, the first SCR catalyst composition promoting higher N 2  formation and lower N 2 O formation than the second SCR catalyst composition, and the second SCR catalyst composition having a different composition than the first SCR catalyst composition, the second SCR catalyst composition promoting lower N 2  formation and higher N 2 O formation than the first SCR catalyst composition. The SCR catalyst systems are useful in methods and systems to catalyze the reduction of nitrogen oxides in the presence of a reductant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119(e)to U.S. Provisional Patent Application No. 61/781,760, filed on Mar. 13,2013, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention pertains to the field of selective catalyticreduction catalysts. More particularly, embodiments of the inventionrelate to selective catalytic reduction catalyst systems comprising afirst SCR catalyst composition and a second SCR catalyst composition, alean burn engine exhaust system, and methods of using these catalystsystems in a variety of processes such as abating pollutants in exhaustgases.

BACKGROUND

Operation of lean burn engines, e.g. diesel engines and lean burngasoline engines, provide the user with excellent fuel economy and havevery low emissions of gas phase hydrocarbons and carbon monoxide due totheir operation at high air/fuel ratios under fuel lean conditions.Diesel engines, in particular, also offer significant advantages overgasoline engines in terms of their durability and their ability togenerate high torque at low speed.

From the standpoint of emissions, however, diesel engines presentproblems more severe than their spark-ignition counterparts. Emissionproblems relating to particulate matter (PM), nitrogen oxides (NO_(x)),unburned hydrocarbons (HC) and carbon monoxide (CO). NO_(x) is a termused to describe various chemical species of nitrogen oxides, includingnitrogen monoxide (NO) and nitrogen dioxide (NO₂), among others. NO isof concern because it is believed to under a process known asphoto-chemical smog formation, through a series of reactions in thepresence of sunlight and hydrocarbons, and NO is a significantcontributor to acid rain. NO₂, on the other hand, has a high potentialas an oxidant and is a strong lung irritant. Particulates (PM) are alsoconnected with respiratory problems. As engine operation modificationsare made to reduce particulates and unburned hydrocarbons on dieselengines, the NO and NO₂ emissions tend to increase.

Effective abatement of NO_(x) from lean burn engines is difficult toachieve because high NO_(x) conversion rates typically requirereductant-rich conditions. Conversion of the NO_(x) component of exhauststreams to innocuous components generally requires specialized NO_(x)abatement strategies for operation under fuel lean conditions

Selective catalytic reduction (SCR), using ammonia or ammonia precursoras reducing agent is believed to be the most viable technique for theremoval of nitrogen oxides from the exhaust of diesel vehicles. Intypical exhaust, the nitrogen oxides are mainly composed of NO (>90%),so the SCR catalyst favors the conversion of NO and NH₃ into nitrogenand water. Two major challenges in developing catalysts for theautomotive application of the ammonia SCR process are to provide a wideoperating window for SCR activity, including low temperatures of from200° C. and higher and improvement of the catalyst's hydrothermalstability for temperatures above 500° C. As used herein hydrothermalstability refers to retention of a material's capability to catalyze theSCR of NO_(x), with a preference for the retention to be at least 85% ofthe material's NO_(x) conversion ability prior to hydrothermal aging.

Metal-promoted zeolite catalysts including, among others, iron-promotedand copper-promoted zeolite catalysts, where, for instance, the metal isintroduced via ion-exchange, for the selective catalytic reduction ofnitrogen oxides with ammonia are known. Iron-promoted zeolite beta hasbeen an effective catalyst for the selective reduction of nitrogenoxides with ammonia. Unfortunately, it has been found that under harshhydrothermal conditions, such as reduction of NO_(x) from gas exhaust attemperatures exceeding 500° C., the activity of many metal-promotedzeolites, such as Cu and Fe versions of ZSM-5 and Beta, begins todecline. This decline in activity is believed to be due todestabilization of the zeolite such as by dealumination and consequentloss of metal-containing catalytic sites within the zeolite.

To maintain the overall activity of NO_(x) reduction, increased levelsof the washcoat loading of the iron-promoted zeolite catalyst must beprovided. As the levels of the zeolite catalyst are increased to provideadequate NO_(x) removal, there is an obvious reduction in the costefficiency of the process for NO_(x) removal as the costs of thecatalyst rise.

In some SCR systems, particularly heavy duty diesel (HDD), controllingsecondary pollutant N₂O emitted from the SCR system has become moreimportant. Additionally, certain existing catalysts, such as copperpromoted zeolites (e.g Cu-SSZ-13), tend to produce unacceptably high N₂Oemissions. Because N₂O is a greenhouse gas and emissions regulations arebecoming increasingly stringent, there is a need for systems that reducethe amount of N₂O emitted from SCR systems.

SUMMARY

One aspect of the invention pertains to a selective catalytic reduction(SCR) catalyst system. In a first embodiment, the system comprises afirst SCR catalyst composition and a second SCR catalyst compositionarranged in the system, the first SCR catalyst composition promotinghigher N₂ formation and lower N₂O formation than the second SCR catalystcomposition, and the second SCR catalyst composition having a differentcomposition than the first SCR catalyst composition, the second SCRcatalyst composition promoting lower N₂ formation and higher N₂Oformation than the first SCR catalyst composition.

In a second embodiment, the first SCR catalyst composition is modifiedso that the first SCR catalyst composition and the second SCR catalystcomposition are disposed on a common substrate.

In a third embodiment, the SCR catalyst system the first or secondembodiments is modified so that the first SCR catalyst composition islocated upstream of the second SCR catalyst composition.

In a fourth embodiment, the SCR catalyst system of the first throughthird embodiments is modified so that the first SCR catalyst compositionand the second SCR catalyst composition are disposed on differentsubstrates.

In a fifth embodiment, the system of the first through fourthembodiments is modified so that first SCR catalyst composition islocated upstream of the second SCR catalyst composition.

In a sixth embodiment, the first or second embodiments are modifiedwherein the first SCR catalyst composition and the second SCR catalystcomposition are in a layered relationship, with the first SCR catalystcomposition layered on top of the second SCR catalyst composition.

In seventh embodiment, any of the first through sixth embodiments, theSCR catalyst system of claim the first SCR catalyst compositioncomprises a mixed oxide.

In an eighth embodiment, seventh embodiment can be modified so that themixed oxide is selected from Fe/titania, Fe/alumina, Mg/titania,Cu/titania, Ce/Zr, vanadia/titania, and mixtures thereof.

In a ninth embodiment, the eighth embodiment is modified so that themixed oxide comprises vanadia/titania.

In a tenth embodiment, the ninth embodiment is modified so that thevanadia/titania is stabilized with tungsten.

In an eleventh embodiment, any of the first through tenth embodimentscan be modified wherein the second SCR catalyst comprises ametal-exchanged 8-ring small pore molecular sieve.

In a twelfth embodiment, the eleventh embodiment can be modified whereinthe molecular sieve has a structure type selected from the groupconsisting of AEI, AFT, AFX, CHA, EAB, ERI, KFI, LEV, SAS, SAT, DDR, andSAV.

In a thirteenth embodiment, the twelfth embodiment is modified whereinthe molecular sieve is an aluminosilicate zeolite and has the CHAstructure type.

In a fourteenth embodiment, the thirteenth embodiment is modifiedwherein the zeolite is selected from SSZ-13 and SSZ-62.

In a fifteenth embodiment, any of the eleventh through fourteenthembodiments can be modified wherein the metal is selected from the groupconsisting of Cu, Fe, Co, Ce and Ni.

In a sixteenth embodiment, the fifteenth embodiment is modified, whereinthe metal is selected from Cu.

In a seventeenth embodiment, the sixteenth embodiment is modified,wherein the zeolite is exchanged with Cu in the range of 2% to 8% byweight.

An eighteenth embodiment pertains to a selective catalytic reduction(SCR) catalyst system comprising a first SCR catalyst compositioncomprising vanadia/titania disposed on a substrate and a second SCRcatalyst composition comprising a metal-exchanged 8-ring small poremolecular sieve disposed on a substrate.

In a nineteenth embodiment, the eighteenth embodiment is modified,wherein molecular sieve has a structure type selected from the groupconsisting of AEI, AFT, AFX, CHA, EAB, ERI, KFI, LEV, SAS, SAT, DDR, andSAV.

In a twentieth embodiment, the nineteenth embodiment is modified,wherein the molecular sieve is an aluminosilicate zeolite and has theCHA structure type.

In a twenty-first embodiment, the twentieth embodiment is modified,wherein the zeolite is selected from SSZ-13 and SSZ-62.

In a twenty-second embodiment, the eighteenth through twenty-firstembodiments are modified, wherein the metal is selected from the groupconsisting of Cu, Fe, Co, Ce and Ni.

In a twenty-third embodiment, the twenty-second embodiment are modified,wherein the metal is selected from Cu.

In a twenty-fourth embodiment the eighteenth through twenty-thirdembodiments are modified, wherein the zeolite is exchanged with Cu inthe range of 2% to 8% by weight.

In a twenty-fifth embodiment, the eighteenth through twenty-fourthembodiments are modified wherein the vanadia/titania is stabilized withtungsten.

In a twenty-sixth embodiment, the eighteenth through twenty-fifthembodiments are modified, wherein the first SCR catalyst composition andsecond SCR catalyst composition are disposed on a common substrate.

In a twenty-seventh embodiment, the eighteenth through twenty-sixthembodiments are modified, wherein the first SCR catalyst composition islocated upstream of the second SCR catalyst composition.

In a twenty-eighth embodiment, the eighteenth through twenty-seventhembodiments are modified, wherein vanadia/titania promotes higher N₂formation and lower N2O formation than the metal-exchanged 8-ring smallpore molecular sieve, and wherein the metal-exchanged 8-ring small poremolecular sieve promotes lower N₂ formation and higher N₂O formationthan the vanadia/titania.

In a twenty-ninth embodiment, the eighteenth through twenty-fifthembodiments are modified, where the first SCR catalyst composition andsecond SCR catalyst composition are disposed on separate substrates.

In a thirtieth embodiment, the twenty ninth embodiment is modified,wherein the first SCR catalyst composition is located upstream of thesecond SCR catalyst composition.

In a thirty-first embodiment, the twenty-sixth embodiment is modifiedwherein the first SCR catalyst composition and the second SCR catalystcomposition are in a layered relationship, with the first SCR catalystcomposition is layered on top of the second SCR catalyst composition.

In a thirty-second embodiment, the thirty-first embodiment is modified,wherein molecular sieve has a structure type selected from the groupconsisting of AEI, AFT, AFX, CHA, EAB, ERI, KFI, LEV, SAS, SAT, DDR, andSAV.

In a thirty-third embodiment, the thirty-second embodiment is modified,wherein the molecular sieve is an aluminosilicate zeolite and has theCHA structure type.

In a thirty-fourth embodiment, the thirty-third embodiment is modified,wherein the zeolite is selected from SSZ-13 and SSZ-62.

In a thirty-fifth embodiment, the thirty-first through thirty-fourthembodiments are modified, wherein the metal is selected from the groupconsisting of Cu, Fe, Co, Ce, and Ni.

In a thirty sixth embodiment, the thirty fifth embodiment is modified,wherein the metal is selected from Cu.

In a thirty seventh embodiment, the thirty-third embodiment is modified,wherein the zeolite is exchanged with Cu.

In a thirty-eighth embodiment, the thirty-first through thirty-seventhembodiments are modified, wherein the vanadia/titania is stabilized withtungsten.

Another aspect of the invention pertains to a lean burn engine exhaustas treatment system. In a thirty-ninth embodiment, a lean burn engineexhaust gas treatment system comprises the catalyst system of any of thefirst through thirty-seventh embodiments, a lean burn engine, and anexhaust gas conduit in fluid communication with the lean burn engine,wherein the catalyst system is downstream of the engine.

In a fortieth embodiment, the thirty-ninth embodiment is modified,wherein the engine is a heavy duty diesel engine.

Another aspect of the invention pertains to a method of removingnitrogen oxides from exhaust gas of a lean burn engine. In a forty-firstembodiment, a method of removing nitrogen oxides from exhaust gas from alean burn engine, the method comprising contacting an exhaust gas streamwith a selective catalytic reduction (SCR) catalyst system including afirst SCR catalyst composition comprising vanadia/titania disposed on asubstrate and a second SCR catalyst composition comprising ametal-exchanged 8-ring small pore molecular sieve disposed on asubstrate.

In a forty-second embodiment, the forty-first embodiment is modified,wherein the exhaust gas comprises NO_(x).

In a forty-third embodiment, the forty-first and forty-secondembodiments are modified, wherein the lean burn engine is a heavy dutydiesel engine.

In a forty-fourth embodiment, a lean burn engine exhaust gas treatmentsystem comprises the catalyst system of the nineteenth embodiment, alean burn engine, and an exhaust gas conduit in fluid communication withthe lean burn engine, wherein the catalyst system is downstream of theengine.

In a forty-fifth embodiment, the forty-fourth embodiment is modified,wherein the engine is a heavy duty diesel engine.

A forty-sixth embodiment pertains to a method of removing nitrogenoxides from exhaust gas from a lean burn engine, the method comprisingcontacting the exhaust gas with selective catalytic reduction (SCR)catalyst system including a first SCR catalyst composition and a secondSCR catalyst composition arranged in the system, the first SCR catalystcomposition promoting higher N₂ formation and lower N₂O formation thanthe second catalyst composition, and the second catalyst compositionhaving a different composition than the first SCR catalyst composition,the second catalyst composition promoting lower N₂ formation and higherN₂O formation than the first SCR catalyst composition.

In a forty-seventh embodiment, the first through thirty seventhembodiments are modified, wherein the second catalyst composition has ahigher NH₃ storage capacity that the first catalyst composition.

In a forty-eighth embodiment, a selective catalytic reduction (SCR)catalyst hybrid system for removing NOx from engine exhaust, the systemcomprises a first SCR catalyst composition and a second SCR catalystcomposition arranged in the system, the first SCR catalyst compositionhaving a faster DeNOx response time when exposed to ammonia than thesecond catalyst composition and the second SCR catalyst composition hasa higher steady state DeNOx performance than the first catalystcomposition and the first SCR catalyst composition provides a targetDeNOx percentage at a lower ammonia storage level than the second SCRcatalyst composition to provide the same DeNOx percentage, and whereinthe system provides higher DeNOx steady state performance than the firstcatalyst composition.

In a forty-ninth embodiment, the forty-eighth embodiment is modified,wherein under acceleration conditions in which sudden increases ofexhaust temperature are produced, ammonia desorbed from the hybridsystem due to the temperature increase is less than ammonia desorbedfrom a system having only the second catalyst composition.

In a fiftieth embodiment, the forty-eight or forty-ninth embodiments aremodified, wherein the first catalyst composition comprisesvanadia/titania stabilized with tungsten.

In a fifty-first embodiment, the fiftieth embodiment is modified,wherein the second catalyst composition comprises a metal-exchanged8-ring small pore molecular sieve.

In a fifty-second embodiment, the fifty-first embodiment is modified,wherein the molecular sieve has a structure type selected from the groupconsisting of AEI, AFT, AFX, CHA, EAB, ERI, KFI, LEV, SAS, SAT, DDR, andSAV.

In a fifty-third embodiment, the fifty-second embodiment is modified,wherein the molecular sieve is an aluminosilicate zeolite and has theCHA structure type.

In a fifty-fourth embodiment, the forty-eighth through fifty-thirdembodiments are modified, wherein the zeolite is selected from SSZ-13and SSZ-62 and the metal comprises Cu.

In a fifty fifth embodiment, the system of the first throughthirty-eighth embodiments are modified wherein the first SCR catalystcomposition promotes higher N₂ formation and lower N₂O formation thanthe second SCR catalyst composition, and the second SCR catalystcomposition promotes lower N₂ formation and higher N₂O formation for atemperature range of 200° C. to 600° C.

In a fifty-sixth embodiment, the forty-eighth through fifty-fourthembodiments are modified, wherein the first SCR catalyst composition hasa faster DeNOx response time when exposed to ammonia than the secondcatalyst composition and the second SCR catalyst composition has ahigher steady state DeNOx performance than the first catalystcomposition and the first SCR catalyst composition provides a targetDeNOx percentage at a lower ammonia storage level than the second SCRcatalyst composition to provide the same DeNOx percentage, and whereinthe system provides higher DeNOx steady state performance than the firstcatalyst composition formation for a temperature range of 200° C. to600° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a partial cross-sectional view of a SCR catalyst systemaccording to one or more embodiments;

FIG. 2 shows a partial cross-sectional view of a SCR catalyst systemaccording to one or more embodiments;

FIG. 3 shows a partial cross-sectional view of a SCR catalyst systemaccording to one or more embodiments;

FIG. 4 is a graph comparing N₂O emissions for a SCR catalyst systemaccording to one or more embodiments and a comparative system;

FIG. 5 is a graph comparing N₂O emissions for a SCR catalyst systemaccording to one or more embodiments and a comparative system;

FIG. 6 is a graph comparing N₂O emissions for a SCR catalyst systemaccording to one or more embodiments and a comparative system, bothsystems with an upstream diesel oxidation catalyst;

FIG. 7 is a graph comparing N₂O emissions for a SCR catalyst systemaccording to one or more embodiments and a comparative system, bothsystems with an upstream diesel oxidation catalyst;

FIG. 8 is a graph comparing NO_(x) conversions for a SCR catalyst systemaccording to one or more embodiments and a comparative system, bothsystems with an upstream diesel oxidation catalyst;

FIG. 9 is a graph comparing NO_(x) conversions after sulfation for a SCRcatalyst system according to one or more embodiments and a comparativesystem, both systems with an upstream diesel oxidation catalyst;

FIG. 10 is a graph generated by a computer model as described in Example6, showing an Analysis of Response Curves-DeNO_(x) vs. Time at 225° C.and 10% NO₂; and

FIG. 11 is a graph generated by a computer model as described in Example6, showing an Analysis of Response Curves-DeNO_(x) vs. Total AbsorbedNH₃ at 225° C. and 10% NO₂.

DETAILED DESCRIPTION

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

Governmental regulations require the use of NO_(x) reductiontechnologies for light and heavy-duty lean burn engine vehicles.Selective catalytic reduction (SCR) of NO_(x) using urea is an effectiveand dominant emission control technology for NO_(x) control. To meetfuture governmental regulations, an SCR catalyst system that hasimproved performance compared to the current Cu-SSZ-13 based systems.Embodiments of the invention pertain to an SCR catalyst system havinglower N₂O emissions and also NO_(x) conversion efficiency improvement atlow NH₃ storage levels than single SCR catalysts and other dual SCRcatalyst systems. Without intending to be bound by theory, it is thoughtthat the dynamic response of the SCR catalyst system according to one ormore embodiments is provided by improved NH₃ storage capacity. Thefeatures of the invention described herein should be provided over theentire SCR temperature range of interest, namely 200° C. to 600° C.According to one or more embodiments, the first and second SCR catalystcompositions exclude platinum group metals such as Pt, Pd and Rh.

Embodiments of the invention are directed to SCR catalyst systems,methods for their preparation, exhaust gas purification systems, andmethods of abating nitrogen oxides from exhaust gases using such SCRcatalyst systems.

Embodiments are directed to the use of SCR catalyst systems providingimproved NO_(x) performance for lean burn engines. While the SCRcatalyst systems can be used in any lean burn engine, in specificembodiments, the catalyst systems are to be used in heavy duty dieselapplications. Heavy duty diesel applications include diesel enginepowered vehicles having a gross vehicle weight rating (GVWR) of above8,500 lbs federally and above 14,000 lbs in California (model year 1995and later). The SCR catalyst systems according to embodiments may haveuse in other engines as well, including, but not limited to, nonroaddiesel engines, locomotives, marine engines, and stationary dieselengines. The invention may have applicability to other lean burn enginestypes as well such as light duty diesel, compressed natural gas and leanburn gasoline direct injected engines.

With respect to the terms used in this disclosure, the followingdefinitions are provided.

As used herein, the term “catalyst” or “catalyst composition” refers toa material that promotes a reaction. As used herein, the phrase“catalyst system” refers to a combination of two or more catalysts, forexample a combination of a first SCR catalyst and a second SCR catalyst.The catalyst system may be in the form of a washcoat in which the twoSCR catalysts are mixed together.

As used herein, the terms “upstream” and “downstream” refer to relativedirections according to the flow of an engine exhaust gas stream from anengine towards a tailpipe, with the engine in an upstream location andthe tailpipe and any pollution abatement articles such as filters andcatalysts being downstream from the engine.

As used herein, the term “stream” broadly refers to any combination offlowing gas that may contain solid or liquid particulate matter. Theterm “gaseous stream” or “exhaust gas stream” means a stream of gaseousconstituents, such as the exhaust of a lean burn engine, which maycontain entrained non-gaseous components such as liquid droplets, solidparticulates, and the like. The exhaust gas stream of a lean burn enginetypically further comprises combustion products, products of incompletecombustion, oxides of nitrogen, combustible and/or carbonaceousparticulate matter (soot), and un-reacted oxygen and nitrogen.

As used herein, the term “substrate” refers to the monolithic materialonto which the catalyst composition is placed, typically in the form ofa washcoat containing a plurality of particles containing a catalyticcomposition thereon. A washcoat is formed by preparing a slurrycontaining a specified solids content (e.g., 30-90% by weight) ofparticles in a liquid vehicle, which is then coated onto a substrate anddried to provide a washcoat layer.

As used herein, the term “washcoat” has its usual meaning in the art ofa thin, adherent coating of a catalytic or other material applied to asubstrate material, such as a honeycomb-type carrier member, which issufficiently porous to permit the passage of the gas stream beingtreated.

“Catalytic article” refers to an element that is used to promote adesired reaction. For example, a catalytic article may comprise awashcoat containing catalytic compositions on a substrate.

In one or more embodiments, the substrate is a ceramic or metal having ahoneycomb structure. Any suitable substrate may be employed, such as amonolithic substrate of the type having fine, parallel gas flow passagesextending there through from an inlet or an outlet face of the substratesuch that passages are open to fluid flow there through. The passages,which are essentially straight paths from their fluid inlet to theirfluid outlet, are defined by walls on which the catalytic material iscoated as a washcoat so that the gases flowing through the passagescontact the catalytic material. The flow passages of the monolithicsubstrate are thin-walled channels, which can be of any suitablecross-sectional shape and size such as trapezoidal, rectangular, square,sinusoidal, hexagonal, oval, circular, etc. Such structures may containfrom about 60 to about 900 or more gas inlet openings (i.e. cells) persquare inch of cross section.

The ceramic substrate may be made of any suitable refractory material,e.g. cordierite, cordierite-α-alumina, silicon nitride, zircon mullite,spodumene, alumina-silica-magnesia, zircon silicate, sillimanite, amagnesium silicate, zircon, petalite, α-alumina, an aluminosilicate andthe like.

The substrates useful for the catalyst compositions of embodiments ofthe present invention may also be metallic in nature and be composed ofone or more metals or metal alloys. The metallic substrates may beemployed in various shapes such as pellets, corrugated sheet ormonolithic form. Specific examples of metallic substrates include theheat-resistant, base-metal alloys, especially those in which iron is asubstantial or major component. Such alloys may contain one or more ofnickel, chromium, and aluminum, and the total of these metals mayadvantageously comprise at least about 15 wt. % of the alloy, forinstance, about 10 to 25 wt. % chromium, about 1 to 8 wt. % of aluminum,and about 0 to 20 wt. % of nickel.

According to a first aspect of the invention, a selective catalyticreduction (SCR) catalyst system comprises a first SCR catalystcomposition and a second SCR catalyst composition arranged in thesystem. In one or more embodiments, the second SCR catalyst compositionhas a different composition than first SCR catalyst composition. Thefirst SCR catalyst composition promotes higher N₂ formation and lowerN₂O formation than the second SCR catalyst composition, while the secondcatalyst composition promotes lower N₂ formation and higher N₂Oformation than the first SCR catalyst composition. To reduce NH₃emissions, in one or more embodiments, the first SCR catalyst shouldhave a lower NH₃ adsorption capacity/desorption temperature than thesecond SCR catalyst composition.

In one or more embodiments, the first SCR catalyst composition and thesecond SCR catalyst composition are on the same or a common substrate.In other embodiments, the first SCR catalyst composition and second SCRcatalyst composition are on separate substrates.

In one embodiment, the first SCR catalyst and the second SCR catalystare arranged in a laterally zoned configuration, with the first catalystupstream from the second catalyst. The upstream and downstream catalystscan be arranged on the same substrate or on different substratesseparated from each other. In another specific embodiment, the first SCRcatalyst and the second SCR catalyst are in a layered arrangement withthe second SCR catalyst being disposed on a substrate and the first SCRcatalyst in a layer overlying the second SCR catalyst. Each of theseembodiments will be described in more detail below.

In specific embodiments, each of the first SCR catalyst composition andsecond SCR catalyst composition is used as a molded catalyst, still morespecifically as a molded catalyst wherein the SCR catalyst compositionis deposited on a suitable refractory substrate, still more specificallyon a “honeycomb” substrate, for the selective reduction of nitrogenoxides NO_(x), i.e. for selective catalytic reduction of nitrogenoxides. According to embodiments of the invention, the SCR catalystcomposition can be in the form of self-supporting catalyst particles oras a honeycomb monolith formed of the SCR catalyst composition.

According to one or more embodiments, the first SCR catalyst compositioncomprises a mixed oxide. As used herein, the term “mixed oxide” refersto an oxide that contains cations of more than one chemical element orcations of a single element in several states of oxidation. In one ormore embodiments, the mixed oxide is selected from Fe/titania (e.g.FeTiO₃), Fe/alumina (e.g. FeAl₂O₃), Mg/titania (e.g. MgTiO₃), Mg/alumina(e.g. MgAl₂O₃), Mn/alumina, Mn/titania (e.g. MnO_(x)/TiO₂) (e.g.MnO_(x)/Al₂O₃), Cu/titania (e.g. CuTiO₃), Ce/Zr (e.g. CeZrO₂), Ti/Zr(e.g. TiZrO₂), vanadia/titania (e.g. V₂O₅/TiO₂), and mixtures thereof.In specific embodiments, the mixed oxide comprises vanadia/titania. Thevanadia/titania oxide can be activated or stabilized with tungsten (e.g.WO₃) to provide V₂O₅/TiO₂/WO₃.

According to one or more embodiments, a first SCR catalyst compositioncomprising vanadia/titania generates significantly less N₂O than zeoliteSCR catalysts, especially under rich NO₂ conditions. In one or moreembodiments, the first SCR catalyst composition comprises titania on towhich vanadia has been dispersed. The vanadia can be dispersed atconcentrations ranging from 1 to 10 wt %, including 1, 2, 3, 4, 5, 6, 7,8, 9, 10 wt %. In specific embodiments the vanadia is activated orstabilized by tungsten (WO₃). The tungsten can be dispersed atconcentrations ranging from 0.5 to 10 wt %, including 1, 2, 3, 3. 4, 5,6, 7, 8, 9, and 10, wt %. All percentages are on an oxide basis.

According to one or more embodiments, the second SCR catalyst comprisesa metal-exchanged molecular sieve. The metal is selected from Cu, Fe,Co, Ni, Ce and Pt. In specific embodiments, the metal is Cu.

As used herein, the term “molecular sieves” refers to materials based onan extensive three-dimensional network of oxygen ions containinggenerally tetrahedral type sites and having a pore distribution.Molecular sieves such as zeolites have been used extensively to catalyzea number of chemical reactions in refinery and petrochemical reactions,and catalysis, adsorption, separation, and chromatography. For example,with respect to zeolites, both synthetic and natural zeolites and theiruse in promoting certain reactions, including conversion of methanol toolefins (MTO reactions) and the selective catalytic reduction (SCR) ofnitrogen oxides with a reductant such as ammonia, urea or a hydrocarbonin the presence of oxygen, are well known in the art. Zeolites arecrystalline materials having rather uniform pore sizes which, dependingupon the type of zeolite and the type and amount of cations included inthe zeolite lattice, range from about 3 to 10 Angstroms in diameter.

Catalyst compositions employed in the SCR process ideally should be ableto retain good catalytic activity over the wide range of temperatureconditions of use, for example, 200° C. to 600° C. or higher, underhydrothermal conditions. Hydrothermal conditions are often encounteredin practice, such as during the regeneration of a soot filter, acomponent of the exhaust gas treatment system used for the removal ofparticles.

According to specific embodiments, the molecular sieves of the secondSCR catalyst composition have 8-ring pore openings and double-six ringsecondary building units, for example, those having the followingstructure types: AEI, AFT, AFX, CHA, EAB, ERI, KFI, LEV, SAS, SAT, DDR,and SAV. According to one or more embodiments, it will be appreciatedthat by defining the molecular sieves by their structure type, it isintended to include the structure type and any and all isotypicframework materials such as SAPO, AlPO and MeAPO materials having thesame structure type.

Zeolites having 8-ring pore openings and double-six ring secondarybuilding units, particularly those having cage-like structures haverecently found interest in use as SCR catalysts. A specific type ofzeolite having these properties is chabazite (CHA), which is a smallpore zeolite with 8 member-ring pore openings (having a pore size in atleast one dimension of less than 4.3 Angstroms, e.g. about 3.8Angstroms) accessible through its 3-dimensional porosity. A cage likestructure results from the connection of double six-ring building unitsby 4 rings.

Metal-promoted, particularly copper-promoted aluminosilicate zeoliteshaving the CHA structure type (e.g., SSZ-13 and SSZ-62) and a silica toalumina molar ratio greater than 1, particularly those having a silicato alumina ratio greater than or equal to 5, 10, or 15 and less thanabout 1000, 500, 250, 100 and 50 have recently solicited a high degreeof interest as catalysts for the SCR of oxides of nitrogen in leanburning engines using nitrogenous reductants. This is because of thewide temperature window coupled with the excellent hydrothermaldurability of these materials, as described in U.S. Pat. No. 7,601,662.Prior to the discovery of metal promoted zeolites described in U.S. Pat.No. 7,601,662, while the literature had indicated that a large number ofmetal-promoted zeolites had been proposed in the patent and scientificliterature for use as SCR catalysts, each of the proposed materialssuffered from one or both of the following defects: (1) poor conversionof oxides of nitrogen at low temperatures, for example 350° C. andlower; and (2) poor hydrothermal stability marked by a significantdecline in catalytic activity in the conversion of oxides of nitrogen bySCR. Thus, the invention described in U.S. Pat. No. 7,601,662 addresseda compelling, unsolved need to provide a material that would provideconversion of oxides of nitrogen at low temperatures and retention ofSCR catalytic activity after hydrothermal aging at temperatures inexcess of 650° C.

Zeolitic chabazite include a naturally occurring tectosilicate mineralof a zeolite group with approximate formula:(Ca,Na₂,K₂,Mg)Al₂Si₄O₁₂.6H₂O (e.g., hydrated calcium aluminum silicate).Three synthetic forms of zeolitic chabazite are described in “ZeoliteMolecular Sieves,” by D. W. Breck, published in 1973 by John Wiley &Sons, which is hereby incorporated by reference. The three syntheticforms reported by Breck are Zeolite K-G, described in J. Chem. Soc., p.2822 (1956), Barrer et al; Zeolite D, described in British Patent No.868,846 (1961); and Zeolite R, described in U.S. Pat. No. 3,030,181,which are hereby incorporated by reference. Synthesis of anothersynthetic form of zeolitic chabazite, SSZ-13, is described in U.S. Pat.No. 4,544,538, which is hereby incorporated by reference. Synthesis of asynthetic form of a molecular sieve having the chabazite crystalstructure, silicoaluminophosphate 34 (SAPO-34), is described in U.S.Pat. No. 4,440,871 and U.S. Pat. No. 7,264,789, which are herebyincorporated by reference. A method of making yet another syntheticmolecular sieve having chabazite structure, SAPO-44, is described inU.S. Pat. No. 6,162,415, which is hereby incorporated by reference.

In more specific embodiments, reference to an aluminosilicate zeolitestructure type limits the material to molecular sieves that do notinclude phosphorus or other metals substituted in the framework. Ofcourse, aluminosilicate zeolites may be subsequently ion-exchanged withone or more promoter metals such as iron, copper, cobalt, nickel, ceriumor platinum group metals. However, to be clear, as used herein,“aluminosilicate zeolite” excludes aluminophosphate materials such asSAPO, ALPO, and MeAPO materials, and the broader term “zeolite” isintended to include aluminosilicates and aluminophosphates. In one ormore embodiments, the molecular sieve can include all aluminosilicate,borosilicate, gallosilicate, MeAPSO, and MeAPO compositions. Theseinclude, but are not limited to SSZ-13, SSZ-62, natural chabazite,zeolite K-G, Linde D, Linde R, LZ-218, LZ-235. LZ-236, ZK-14, SAPO-34,SAPO-44, SAPO-47, ZYT-6, CuSAPO-34, CuSAPO-44, and CuSAPO-47.

In one or more embodiments, the molecular sieve of the second SCRcatalyst composition has a structure type selected from the groupconsisting of AEI, AFT, AFX, CHA, EAB, ERI, KFI, LEV, SAS, SAT, DDR, andSAV. In a specific embodiment, the molecular sieve is an aluminosilicatezeolite and has the CHA structure type, for example SSZ-13 or SSZ-62. Inanother specific embodiment, the molecular sieve is an aluminosilicatezeolite and has the AEI structure type, for example SSZ-39.

In specific embodiments, the 8-ring small pore molecular sieve promotedwith copper has a mole ratio of silica to alumina greater than about 15,even more specifically greater than about 20. In specific embodiments,the 8-ring small pore molecular sieve promoted with copper has a moleratio of silica to alumina in the range from about 20 to about 256, morespecifically in the range from about 25 to about 40.

In specific embodiments, the atomic ratio of copper to aluminum exceedsabout 0.25. In more specific embodiments, the ratio of copper toaluminum is from about 0.25 to about 1, even more specifically fromabout 0.25 to about 0.5. In even more specific embodiments, the ratio ofcopper to aluminum is from about 0.3 to about 0.4.

In general, the SCR catalyst system according to one or more embodimentsshould exhibit both good low temperature NO_(x) conversion activity(NO_(x) conversion >50% at 200° C.) and good high temperature NO_(x)conversion activity (NO_(x) conversion >70% at 450° C.). The NO_(x)activity is measured under steady state conditions at maximum NH₃-slipconditions in a gas mixture of 500 ppm NO, 500 ppm NH₃, 10% O₂, 5% H₂O,balance N₂ at a volume-based space velocity of 80,000 h⁻¹.

According to one or more embodiments, to reduce NH₃ emissions, the firstSCR catalyst composition should have a lower NH₃ adsorption/desorptiontemperature than the second SCR catalyst composition.

According to one or more embodiments, the second SCR catalystcomposition comprises a metal-exchanged 8-ring small pore molecularsieve. In other words, the second SCR catalyst composition is an 8-ringsmall pore molecular sieve that is promoted with a metal. In one or moreembodiments, the metal can be selected from the group consisting of Cu,Fe, Co, Ce, and Ni. In a specific embodiment, the metal is selected fromCu.

Wt % of Promoter Metal:

The promoter metal (e.g. Cu) content of the metal-exchanged 8-ring smallpore molecular sieve, calculated as the metal oxide, in specificembodiments is at least about 2 wt.-%, even more specifically at leastabout 2.5 wt.-% and in even more specific embodiments at least about 3wt.-%, reported on a volatile-free basis. In even more specificembodiments, the metal (e.g. Cu) content of the metal-exchange 8-ringsmall pore molecular sieve, calculated as the metal oxide, is in therange of up to about 8 wt.-%, based on the total weight of the calcinedmolecular sieve reported on a volatile free basis. Therefore, inspecific embodiments, ranges of the 8-ring small pore molecular sievepromoted with a metal selected from Cu, Fe, Co, Ce, and Ni, calculatedas the metal oxide, are from about 2 to about 8 wt.-%, more specificallyfrom about 2 to about 5 wt.-%, and even more specifically from about 2.5to about 3.5 wt.-%, in each case reported on an oxide basis.

In one or more embodiments, the first SCR catalyst and the second SCRcatalyst are arranged in a laterally zoned configuration, with the firstcatalyst upstream from the second catalyst. As used herein, the term“laterally zoned” refers to the location of the two SCR catalystsrelative to one another. Lateral means side-by-side such that the firstSCR catalyst composition and the second SCR catalyst composition arelocated one beside the other with the first SCR catalyst compositionupstream of the second SCR catalyst composition. According to one ormore embodiments, the laterally zoned first and second SCR catalysts canbe arranged on the same or a common substrate or on different substratesseparated from each other.

According to one or more embodiments, the vanadia/titania and themetal-exchanged 8-ring small pore molecular sieve are disposed on acommon or the same substrate. In other embodiments, the vanadia/titaniaand the metal-exchanged 8-ring small pore molecular sieve are disposedon separate substrates. Whether on the same substrate or on differentsubstrates, according to one or more embodiments, the vanadia/titania islocated upstream of the metal-exchanged 8-ring small pore molecularsieve.

In one or more embodiments, the vandia/titania promotes higher N₂formation and lower N₂O formation than the metal-exchanged 8-ring smallpore molecular sieve, and the metal-exchanged 8-ring small poremolecular sieve promotes lower N₂ formation and higher N₂O formationthan the vanadia/titania.

Compositions used commercially, especially in mobile applications,comprise TiO₂ on to which WO₃ and V₂O₅ have been dispersed atconcentrations ranging from 5 to 20 wt. % and 0.5 to 6 wt. %,respectively. These catalysts may contain other inorganic materials suchas SiO₂ and ZrO₂ acting as binders and promoters.

Referring to FIG. 1, an exemplary embodiment of a laterally spacedsystem is shown. The SCR catalyst system 10 is shown in a laterallyzoned arrangement where the first SCR catalyst composition 18 is locatedupstream of the second SCR catalyst composition 20 on a common substrate12. The substrate 12 has an inlet end 22 and an outlet end 24 definingan axial length L. In one or more embodiments, the substrate 12generally comprises a plurality of channels 14 of a honeycomb substrate,of which only one channel is shown in cross-section for clarity. Thefirst SCR catalyst composition 18 extends from the inlet end 22 of thesubstrate 12 through less than the entire axial length L of thesubstrate 12. The length of the first SCR catalyst composition 18 isdenoted as first zone 18 a in FIG. 1. The first SCR catalyst composition18 can, in specific embodiments comprise vanadia/titania. The second SCRcatalyst composition 20 can, in specific embodiments, comprise ametal-exchanged 8-ring small pore molecular sieve. The second SCRcatalyst composition 20 extends from the outlet end 24 of the substrate12 through less than the entire axial length L of the substrate 12. Thelength of the second catalyst composition is denoted as the second zone20 b in FIG. 1. The SCR catalyst system 10 is effective for theselective catalytic reduction of NO_(x).

It will be appreciated that length of the first zone and the second zonecan be varied. In one or more embodiments, the first zone and secondzone can be equal in length. In other embodiments, the first zone can be20%, 25%, 35% or 40%, 60%, 65%, 75% or 80% of the length L of thesubstrate, with the second zone respectively covering the remainder ofthe length L of the substrate.

Referring to FIG. 2, another embodiment of a laterally zoned SCRcatalyst system 110 is shown. The SCR catalyst system 110 shown is alaterally zoned arrangement where the first SCR catalyst composition 118is located upstream of the second SCR catalyst composition 120 onseparate substrates 112 and 113. The first SCR catalyst composition 118is disposed on a substrate 112, and the second SCR catalyst compositionis disposed on a separate substrate 113. The substrates 112 and 113 canbe comprised of the same material or a different material. The substrate112 has an inlet end 122 a and an outlet end 124 a defining an axiallength L1. The substrate 113 has an inlet end 122 b and an outlet end124 b defining an axial length L2. In one or more embodiments, thesubstrates 112 and 113 generally comprise a plurality of channels 114 ofa honeycomb substrate, of which only one channel is shown incross-section for clarity. The first SCR catalyst composition 118extends from the inlet end 122 a of the substrate 112 through the entireaxial length L1 of the substrate 112 to the outlet end 124 a. The lengthof the first SCR catalyst composition 118 is denoted as first zone 118 ain FIG. 2. The first SCR catalyst composition 118 can, in specificembodiments, comprise vanadia/titania. The second SCR catalystcomposition 120 can, in specific embodiments, comprise a metal-exchanged8-ring small pore molecular sieve. The second SCR catalyst composition120 extends from the outlet end 124 b of the substrate 113 through theentire axial length L2 of the substrate 113 to the inlet end 122 b. Thesecond catalyst composition 120 defines a second zone 120 a. The SCRcatalyst system 110 is effective for the selective catalytic reductionof NO_(x). The length of the zones 118 a and 120 a can be varied asdescribed with respect to FIG. 1.

One or more embodiments of the present invention are directed to aselective catalytic reduction (SCR) catalyst system comprising a firstSCR catalyst composition comprising vanadia/titania disposed on asubstrate and a second SCR catalyst composition comprising ametal-exchanged 8-ring small pore molecular sieve disposed on asubstrate, wherein the first SCR catalyst composition and the second SCRcatalyst composition are in a layered arrangement or relationship. Inone or more embodiments, the first SCR catalyst composition is layeredon top of the second SCR catalyst composition.

According to one or more embodiments, the second SCR catalystcomposition is washcoated onto a substrate, and then the first SCRcatalyst composition is washcoated in a layer overlying the second SCRcatalyst composition. In one or more embodiments, the layering isdesigned to optimize the first catalyst composition/second catalystcomposition dry gain for a desirable balance between the benefits ofacting as a protective shield and the potential drawbacks of diffusionbarrier increase. Under low temperatures for extended operations, sulfuris a major concern for Cu-CHA catalysts. In comparison, vanadia/titania(V₂O₅/TiO₂) SCR catalysts are known for having superior sulfurtolerance.

The first and second SCR catalyst compositions can include thecompositions as described above.

Referring to FIG. 3, an exemplary embodiment of a layered SCR catalystsystem 210 is shown. The SCR catalyst system can be in a layeredarrangement where the first SCR catalyst composition 218 is layered ontop of the second SCR catalyst composition 220 on a common substrate212. The substrate 212 has an inlet end 222 and an outlet end 224defining an axial length L3. In one or more embodiments, the substrate212 generally comprises a plurality of channels 214 of a honeycombsubstrate, of which only one channel is shown in cross-section forclarity. The first SCR catalyst composition 218 extends from the inletend 222 of the substrate 212 through the entire axial length L3 of thesubstrate 212 to the outlet end 224. The length of the first SCRcatalyst composition 218 is denoted as 218 a in FIG. 3. The first SCRcatalyst composition 218 can, in specific embodiments, comprisevanadia/titania. The second SCR catalyst composition 220 can, inspecific embodiments, comprise a metal-exchanged 8-ring small poremolecular sieve. The second SCR catalyst composition 220 extends fromthe outlet end 224 of the substrate 212 through the entire axial lengthL3 of the substrate 212 to the outlet end 224. The SCR catalyst system210 is effective for the selective catalytic reduction of NO_(x).

It will be appreciated that the thickness of the layer 218 can berelatively thin compared to the thickness of the layer 220. Thethickness of layer 218 can be sufficiently thick to form a protectiveovercoat on layer 220 to protect the catalyst composition of layer 220from sulfation. In one embodiment, the thickness of the first catalystcomposition layer 218 is 5-10% of the overall thickness of the compositelayer 218 and 220. In other embodiments, the thickness of the firstcatalyst composition layer is 20-30% of the overall thickness of thecomposite layer 218 and 220. In some embodiments, the thickness of thefirst catalyst composition layer is 30-40% of the overall thickness ofthe composite layer 218 and 220.

Exhaust Gas Treatment System:

In one aspect of the invention, exhaust gas treatment system comprises alean burn engine, and exhaust gas conduit in fluid communication withthe lean burn engine, and a selective catalytic reduction catalystsystem including a first SCR catalyst composition and a second SCRcatalyst composition arranged in the system according to one or moreembodiments. In specific embodiments, the lean burn engine is a heavyduty diesel engine.

In one or more embodiments, the exhaust gas treatment system includes anexhaust gas stream containing a reductant such as ammonia, urea and/orhydrocarbon, and in specific embodiments, ammonia and/or urea. Inspecific embodiments, the exhaust gas treatment system further comprisesa second exhaust gas treatment component, for example, a soot filter ora diesel oxidation catalyst.

The soot filter, catalyzed or non-catalyzed, may be upstream ordownstream of the SCR catalyst system according to one or moreembodiment. The diesel oxidation catalyst in specific embodiments islocated upstream of the SCR catalyst system according to one or moreembodiments. In specific embodiments, the diesel oxidation catalyst andthe catalyzed soot filter are upstream from the SCR catalyst system.

In specific embodiments, the exhaust is conveyed from the lean burnengine to a position downstream in the exhaust system, and, in morespecific embodiments, containing NO_(x), where a reductant is added andthe exhaust stream with the added reductant is conveyed to the SCRcatalyst system according to one or more embodiments.

In specific embodiments, the soot filter comprises a wall-flow filtersubstrate, where the channels are alternately blocked, allowing agaseous stream entering the channels from one direction (inletdirection), to flow through the channel walls and exit from the channelsfrom the other direction (outlet direction).

An ammonia oxidation catalyst may be provided downstream of the SCRcatalyst system to remove any slipped ammonia from the system. Inspecific embodiments, the AMOX catalyst may comprise a platinum groupmetal such as platinum, palladium, rhodium or combinations thereof. Inmore specific embodiment, the AMOX catalyst can include a washcoatcontaining SCR catalyst system including a first SCR catalystcomposition disposed on a substrate and a second SCR catalystcomposition disposed on a substrate.

AMOX and/or SCR catalyst composition can be coated on the flow throughor wall-flow filter. If a wall flow substrate is utilized, the resultingsystem will be able to remove particulate matter along with gaseouspollutants. The wall-flow filter substrate can be made from materialscommonly known in the art, such as cordierite, aluminum titanate orsilicon carbide. It will be understood that the loading of the catalyticcomposition on a wall flow substrate will depend on substrate propertiessuch as porosity and wall thickness, and typically will be lower thanloading on a flow through substrate.

SCR Activity:

The invention is now described with reference to the following examples.Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

EXAMPLES Example 1—Preparation of Catalyst Materials Vanadia-TitaniaCatalyst

A standard vanadia/titania/tungsten (V₂O₅ (2.5%)/WO₃ (10%)/TiO₂)catalyst was prepared and a slurry was made at about 30-40% solids bymilling to provide a washcoat slurry.

Cu-Zeolite

A CuCHA (SSZ-13) powder catalyst was prepared by mixing 100 g of Na-formCHA, having a silica/alumina mole ratio of 30, with 400 mL of a copper(II) acetate solution of about 1.0 M. The pH was adjusted to about 3.5with nitric acid. An ion-exchange reaction between the Na-form CHA andthe copper ions was carried out by agitating the slurry at about 80° C.for about 1 hour. The resulting mixture was then filtered to provide afilter cake, and the filter cake was washed with deionized water inthree portions until the filtrate was clear and colorless, and thewashed sample was dried.

The obtained CuCHA catalyst comprised CuO at a range of about 2.5 to3.5% by weight, as determined by ICP analysis. A CuCHA slurry wasprepared to 40% target solids. The slurry was milled and a binder ofzirconium acetate in dilute acetic acid (containing 30% ZrO₂) was addedinto the slurry with agitation.

Example 2—Catalyst System Laterally Zoned

The slurries described above were separately coated onto 12″D×6″Lcellular ceramic substrate having a cell density of 400 cpsi (cells persquare inch) and a wall thickness of 4 mil. The coated substrates weredried at 110° C. for 3 hours and calcined at about 400° C. for 1 hour.The coating process was repeated once to obtain a target washcoatloading of in the range of 3 g/in³ on the vanadia-titania coated core,and 2.1 g/in³ on the CuCHA coated core. The samples were aged for 200hours at 550° C. on a heavy duty diesel engine test cell.

Comparative Example 3—Catalyst System Laterally Zoned

Example 2 was repeated except both substrates were coated with CuCHA atthe same loading.

Example 4—Engine Testing of Laterally Zoned Systems

The catalyst systems in Example 3 and 4 were tested out on a 9 L heavyduty engine together with a motoring electric dynamometer. The testbench is capable of running both steady-state and transient test cycles.In the current work, both a Heavy duty transient test cycle (HDTP) and anon-road transient test cycle (NRTC) were run. Catalysts samples werefull size 12″ diameters parts (400/4), which were 200 h-550° C. engineaged prior to evaluations. To demonstrate the advantage of the lateralzoned system of a 12″×6″ V-SCR brick upstream of a 12″×6″ Cu-CHA brick,a reference sequential 12″×6″ Cu+12″×6″ Cu SCR system was alsoevaluated. In such a comparative study, only the first SCR catalystbrick were switched between V-SCR and Cu-SCR, other systems such as thesecond SCR brick, urea injection system, sample probing locations werekept the same.

During evaluation tests, two MKS FTIR samplers were positioned at SCRupstream and downstream, respectively, for gaseous emissionsmeasurements, including, but not limited to, NO, NO2, and N2O etc.Exhaust sampling lines were heated at constant 190° C. All evaluationtests in this Example were run with ULSD (ultra low sulfur diesel) fuelwhere sulfur concentration is less than 15 ppm (wt %).

In one configuration, a diesel oxidation catalyst and catalyzed sootfilter were placed upstream of the SCR catalyst system to simulate aheavy duty engine transient cycle. In another configuration, the SCRcatalyst system was tested without upstream catalysts or filters.

FIG. 4 shows the results from the HDTP cycle and FIG. 5 shows theresults from the NRTC cycle. Both tests showed the significant reductionin N₂O emission for the samples in which the vanadia-titania catalystwas placed upstream of the Cu-zeolite sample.

The tests were repeated with an upstream diesel oxidation catalyst andcatalyzed soot filter. FIG. 6 shows the results for the HDTP cycle, andFIG. 7 shows the results for the NRTC. Again, the system with thevanadia-titania catalyst upstream of the Cu Zeolite system showed muchlower N₂O emissions.

Example 5—Preparation of Layered Catalyst System

Washcoats from Example 1 were utilized and coated onto a singlesubstrate in a layered configuration as described with respect to FIG.3. The layering was varied as follows for the following samples.

Comparative Sample 5A CuCHA Single Coat 2.1 g/in³

Comparative Sample 5B Bottom Coat CuCHA 2.1 g/in³; Top Coat 0.2 g/in³Titania

Sample 5C CuCHA Bottom Coat—CuCHA 2.1 g/in³; Top Coat 0.1 g/in³ VanadiaTitania

Sample 5D CuCHA Bottom Coat 2.1 g/in³; Top Coat 0.2 g/in³ VanadiaTitania

Sample 5E CuCHA Bottom Coat 2.1 g/in³; Top Coat 0.5 g/in³Vanadia-Titania

Sample 5F CuCHA Bottom Coat 2.1 g/in3; Top Coat 1 g/in³ Vanadia Titania

Example 6—Testing of Layered System

Nitrogen oxides selective catalytic reduction (SCR) efficiency andselectivity of a fresh catalyst core was measured by adding a feed gasmixture of 500 ppm of NO, 500 ppm of NH₃, 10% O₂, 5% H₂O, balanced withN₂ to a steady state reactor containing a 1″D×3″L catalyst core. Thereaction was carried at a space velocity of 80,000 hr⁻¹ across a 150° C.to 460° C. temperature range.

The samples were aged in the presence of 10% H₂O at 550° C. for 4 hours,followed by measurement of the nitrogen oxides SCR efficiency andselectivity by the same process as outlined above for the SCR evaluationon a fresh catalyst core.

Nitrogen oxides selective catalytic reduction (SCR) efficiency andselectivity of a fresh catalyst core was measured by adding a feed gasmixture of 500 ppm of NO, 500 ppm of NH₃, 10% O₂, 5% H₂O, balanced withN₂ to a steady state reactor containing a 1″D×3″L catalyst core. Thereaction was carried at a space velocity of 80,000 hr⁻¹ across a 150° C.to 460° C. temperature range.

Samples prepared as above were tested for SCR performance. In addition,all of the samples except for 5F were exposed to sulfur (sulfation) at300° C. at 20 ppm SO₂ and 5% H₂O and 10% O₂ in a feed gas upstream of aDOC core with the SCR catalysts downstream for 6 hours.

FIG. 8 shows the NO_(x) conversion versus temperature for samples 5A-Fbefore sulfation and FIG. 9 shows NOx conversion versus temperatureafter sulfation. The fresh conversions were comparable for all samples,except for sample 5F. For the sulfated sample, FIG. 9 shows that sample5E had significantly better NO_(x) conversion.

Example 9—Dynamic Response Modeling

FIGS. 10 and 11 illustrate the improvements in dynamic response behaviorof a system according to one or more embodiments. FIGS. 10 and 11 wereprepared using a computer model. Lab reactor and engine lab DeNO_(x)performance measurements to describe the performance of the individualcomponents within the system are the input for the computer model used.The example in FIG. 10 shows the DeNO_(x) performance as a function oftime obtained with fresh systems without ammonia stored prior to thestart of the simulation/urea dosing. A Cu-SSZ13 system and a Vanadiabased SCR system are compared with the Vanadia/Cu-SSZ-13 hybrid system.The Vanadia based SCR catalyst was placed in front of the Cu-SSZ13catalyst with a 50/50 size ratio within the modeled hybrid system. Lowtemperature operation at 225° C. exhaust temperature and 50000 l/h spacevelocity at 500 ppm NO_(x) inlet concentration at an NO₂/NO_(x) ratio of10% was used for the comparison. These SCR inlet conditions can be seenas being typical for systems operated in engine applications with a lowprecious metal loading on an oxidation system in front of the SCR or inSCR only systems. The NSR was chosen at 1.1 in order to reach relativelyfast the maximum DeNO_(x) performance of the systems studied. Althoughthe Cu-SSZ13 system reaches higher DeNO_(x) performances after 700 sec.of dosing, the DeNO_(x) response behavior after start of dosing at 0sec. has a different ranking. The response of the Vanadia based SCRsystem is faster relative to the DeNO_(x) increase after start of dosingcompared with the Cu-SSZ13 system (e.g. up to 350 sec.). The hybridsystem Vanadia-based SCR in combination with the Cu-SSZ13 has theadvantage of being close to the dynamic response behavior of theVanadia-based SCR and additionally delivering higher steady stateDeNO_(x) performances as indicated in FIG. 10 after, for example, 1000sec.

FIG. 11 was generated by re-plotting FIG. 10 by using the total adsorbedNH₃ on the catalysts in grams as the x-axis results. The practicaladvantage of the hybrid system can be seen when comparing the necessaryammonia stored on the catalysts to reach e.g. 70% DeNO_(x). The Cu-SSZ13system needs approximately 4.5 g NH₃, while the Vanadia-based systemwould need approximately 2.5 g, and the proposed hybrid systemapproximately 3 g ammonia stored. The hybrid system therefore woulddeliver DeNO_(x) performance faster and at lower NH₃ storage levelscompared with the Cu-SSZ13 SCR system. Furthermore the hybrid systemwould deliver higher DeNO_(x) steady state performance compared with theVanadia based SCR system. The higher DeNO_(x) performance reached atlower NH₃ storage levels has a further advantage when the engineaccelerates with sudden increases in exhaust temperature. In this case,the amount of ammonia desorbed from the catalysts due to the temperatureincrease is less for the hybrid system compared with the Cu-SSZ13 systemand therefore would result into lower NH₃ slip values behind the SCRportion of the aftertreatment system. Even when using an ammoniaoxidation catalyst is used to control the NH₃ slip coming from the SCR,very high ammonia peaks from acceleration events are often issues forthe ammonia oxidation catalyst due to the typical volumes installed incombination with the ammonia light-off characteristics.

Reference throughout this specification to “one embodiment,” “certainembodiments,” “one or more embodiments” or “an embodiment” means that aparticular feature, structure, material, or characteristic described inconnection with the embodiment is included in at least one embodiment ofthe invention. Thus, the appearances of the phrases such as “in one ormore embodiments,” “in certain embodiments,” “in one embodiment” or “inan embodiment” in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, materials, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It will be apparent to those skilled in the art thatvarious modifications and variations can be made to the method andapparatus of the present invention without departing from the spirit andscope of the invention. Thus, it is intended that the present inventioninclude modifications and variations that are within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A selective catalytic reduction (SCR) catalystsystem comprising a first SCR catalyst composition and a second SCRcatalyst composition arranged in the system, the first SCR catalystcomposition promoting higher N₂ formation and lower N₂O formation thanthe second SCR catalyst composition, and the second SCR catalystcomposition having a different composition than the first SCR catalystcomposition, the second SCR catalyst composition promoting lower N₂formation and higher N₂O formation than the first SCR catalystcomposition, wherein the first SCR catalyst composition is in the formof a first washcoat; wherein the second SCR catalyst composition is inthe form of a second washcoat; wherein the first washcoat and the secondwashcoat are coated on one or more substrates in a laterally zonedconfiguration such that the first SCR catalyst composition is locatedlaterally upstream of the second SCR catalyst composition according to aflow direction of an engine exhaust gas stream from an engine towards atailpipe; wherein the first SCR catalyst composition comprises a mixedoxide comprising vanadia/titania; and wherein the second SCR catalystcomposition comprises a metal-exchanged zeolite, excluding Cu-exchangedor Fe-exchanged ZSM-5 or Beta zeolites; wherein the SCR catalyst systemprovides an improved NOx conversion efficiency as compared to a SCRcatalyst system comprising the first SCR catalyst composition andsubstantially free of the second SCR catalyst composition; and whereinthe SCR catalyst system provides a lower N₂O formation as compared to aSCR catalyst system comprising the second SCR catalyst composition andsubstantially free of the first SCR catalyst composition.
 2. The SCRcatalyst system of claim 1, wherein the first SCR catalyst compositionand the second SCR catalyst composition are disposed on a commonsubstrate.
 3. The SCR catalyst system of claim 1, wherein the first SCRcatalyst composition and the second SCR catalyst composition aredisposed on different substrates.
 4. The SCR catalyst system of claim 1,wherein the vanadia/titania is stabilized with tungsten.
 5. The SCRcatalyst system of claim 1, wherein the zeolite is a metal-exchanged8-ring small pore zeolite.
 6. The SCR catalyst system of claim 5,wherein the zeolite has a structure type selected from the groupconsisting of AEI, AFT, AFX, CHA, EAB, ERI, KFI, LEV, SAS, SAT, DDR, andSAV.
 7. The SCR catalyst system of claim 5, wherein the zeolite is analuminosilicate zeolite and has the CHA structure type.
 8. The SCRcatalyst of claim 7, wherein the zeolite is selected from SSZ-13 andSSZ-62.
 9. The SCR catalyst system of claim 5, wherein the metal isselected from the group consisting of Cu, Fe, Co, Ce and Ni.
 10. The SCRcatalyst system of claim 9, wherein the metal is Cu.
 11. The SCRcatalyst system of claim 7, wherein the zeolite is exchanged with Cu inthe range of 2% to 8% by weight.
 12. A selective catalytic reduction(SCR) catalyst system comprising a first SCR catalyst compositioncomprising vanadia/titania disposed on a substrate and a second SCRcatalyst composition comprising a metal-exchanged 8-ring small poremolecular sieve disposed on a substrate, wherein the first SCR catalystcomposition is in the form of a first washcoat; wherein the second SCRcatalyst composition is in the form of a second washcoat; wherein thefirst washcoat and the second washcoat are coated on one or moresubstrates in a laterally zoned configuration such that the first SCRcatalyst composition is located laterally upstream of the second SCRcatalyst composition according to a flow direction of an engine exhaustgas stream from an engine towards a tailpipe; wherein themetal-exchanged 8-ring small pore molecular sieve is a metal-exchangedzeolite, excluding Cu-exchanged or Fe-exchanged ZSM-5 or Beta zeolites;wherein the SCR catalyst system provides an improved NOx conversionefficiency as compared to a SCR catalyst system comprising the first SCRcatalyst composition and substantially free of the second SCR catalystcomposition; and wherein the SCR catalyst system provides a lower N₂Oformation as compared to a SCR catalyst system comprising the second SCRcatalyst composition and substantially free of the first SCR catalystcomposition.
 13. The SCR catalyst system of claim 12, wherein the secondSCR catalyst composition comprises Cu and an aluminosilicate zeolitehaving the CHA structure type.
 14. A lean burn engine exhaust gastreatment system comprising the SCR catalyst system of claim 1, a leanburn engine, and an exhaust gas conduit in fluid communication with thelean burn engine, wherein the SCR catalyst system is downstream of theengine.
 15. The system of claim 14, wherein the engine is a heavy dutydiesel engine.
 16. A method of removing nitrogen oxides from exhaust gasfrom a lean burn engine, the method comprising contacting an exhaust gasstream the SCR catalyst system of claim
 1. 17. The method of claim 16,wherein the lean burn engine is a heavy duty diesel engine.
 18. The SCRcatalyst system of claim 13, wherein the zeolite is selected from SSZ-13and SSZ-62.
 19. The SCR catalyst system of claim 12, whereinvanadia/titania promotes higher N₂ formation and lower N₂O formationthan the metal-exchanged 8-ring small pore molecular sieve, and whereinthe metal-exchanged 8-ring small pore molecular sieve promotes lower N₂formation and higher N₂O formation than the vanadia/titania and the8-ring small pore molecular sieve has a higher ammonia storage capacitythan the vanadia/titania.
 20. The SCR catalyst system of claim 1,wherein the first SCR catalyst composition and the second SCR catalystcomposition are disposed on a single flow-through substrate, or whereinthe first SCR catalyst composition and the second SCR catalystcomposition are disposed on separate flow-through substrates.